Electro-mechanical devices, such as motors, alternators, and electrical generators, as examples, may require that power be delivered from a stationary power source to a rotating member or that electrical power generated from a rotating member be fed back into the grid or provided to some other stationary structure. Slip rings, brushes, and commutator/brush assemblies are known in the field of electrical motors and generators, and are frequently used to provide electrical coupling between stationary and rotating components of such motors and/or generators.
A brush is a device which conducts current between stationary wires and moving parts, most commonly in a rotating shaft. A brush operates by providing a sliding contact between a stationary source and the rotating surface to which power is provided. There are numerous problems with brush technologies, for example, wear and contact bounce. Contact bounce may result in intermittent loss of physical contact between the brush and moving structure, causing a momentary open circuit. This may cause electrical arcing which may ablate the surface of the brush, thereby shortening its lifespan, especially in the case of high-current brushes and/or highly inductive loads.
A conventional solution to the brush problem is to turn to a brushless motor (or brushless generator). In general, brushless motors/generators may use one or more permanent magnets at the rotor in place of an electromagnet to obviate the need for a brush type device to provide electrical current to the rotor. In the case of high-performance brushless motors/generators, rare earth permanent magnets may be used. However, the reliance on rare earth magnets to generate the desired magnetic field distribution in the rotor assembly may be a significant shortcoming because rare earth magnets are expensive and their cost continues to grow. There is also growing international concern about the environmental impact of rare earth mining and extraction operations.